Conventional IGCC carbon capture methods: The typical process for CO2 capture in an integrated gasification combined cycle (IGCC) plant is well known in the industry. Coal is gasified with O2 at high pressures to produce syngas that is then scrubbed, cleaned, and stripped of CO2 before it is combusted in a gas turbine. Typically, the CO2 is removed from the syngas stream via a physical sorbent like Selexol® or Rectisol®.
The syngas is contacted with lean solvent in a scrubber column where the sorbent absorbs the CO2 and is regenerated by flashing CO2 at successively lower and lower pressures. The low pressure CO2 is compressed to supercritical pressure for sequestration. In this scenario, the CO2 sequestration cost is high because Selexol®-type plants have high capital costs and the CO2 must be pressurized to greater than about 2000 psia from pressures as low as 18 psia.
Carbon capture in IGCC plants with hydrogen membranes: A different approach for syngas processing is to use a hydrogen transport membrane (HTM) to remove H2 from the syngas and leave a CO2-rich stream that can be purified and compressed to supercritical pressure from near-gasifier (e.g., about 350 to 1000 psia) pressure. One way to minimize membrane capital and fuel compression costs in this scenario is to use compressed ASU N2 as a permeate sweep stream to provide a H2—N2 fuel mixture for a gas turbine feed. Compressed N2 sweep decreases H2 permeate partial pressure and minimizes the required membrane area. It also eliminates the need to compress the gas turbine fuel, reduces NOx emissions, and improves gas turbine performance. This approach is not currently used in industrial practice and it is described in more detail below. While an IGCC plant with carbon capture that utilizes HTM technology may be cost competitive with plants that employ traditional carbon capture methods, there are several issues that may arise in these plant designs. For example, even at high syngas H2 recovery, there is a significant amount of H2 in the membrane retentate that must be removed from the CO2-rich stream for it to be sequestered. Costs related to CO2 purification and compression increase with increasing H2 in the CO2-rich stream.
In addition, in an IGCC plant that produces both electricity and H2, there is often not enough heat in the gas turbine exhaust to superheat the steam that is generated in the IGCC process. It is therefore necessary to fire supplemental fuel in the heat recovery steam generator (HRSG) duct burner to superheat the steam for expansion in the steam turbine.